This invention relates generally to the formation of packages that contain at least two discrete and isolated circuits.
As the use of multi-chip and hybrid packages increases, there are increased pressures to include two or more isolated circuits in a single package. When one of the circuits is a relatively high voltage circuit such as might be found in high voltage power supplies, amplifier circuits and the like, the isolated circuits must be spaced apart by specified distances both internally and externally to prevent arcing resulting in short circuits thus posing a fire hazard. By way of example, Underwriters Laboratories (UL) has set forth guidelines as to the minimum space between internal isolated circuits and external exposed surfaces for relatively high voltage devices. As is well known to those skilled in the art, there are a number of standard form factors for integrated circuit packages. Thus, even when a package contains two isolated circuits, it is often desirable to package these circuits in standard small form factor packages such as dual in-line (DIP) or small outline (SO) packages. Referring initially to FIG. 1a, a top view of the external portion of a 24-pin SO package for an Isolation Amplifier manufactured by Burr Brown of Tucson, Ariz. is diagrammatically shown. As can be seen, the external pins relating to the primary circuit 4 and secondary circuit 6 are located a distance d.sub.ext apart.
There are a number of known processes for packaging integrated circuits. In one standard process, a lead frame 10 is formed from a sheet of conductive material (usually via stamping or etching). A die (or multiple dies or components in the case of multi-chip packages) is then attached and electrically connected to the lead frame. By way of example, bonding wires are often used to form the electrical connections. Thereafter, the die(s), a portion of the lead frame, the electrical connections and any other components are then encapsulated in a package. By way of example, overmolding using a plastic material is common, as are the use of ceramic and other packages. A popular and cost effective way of facilitating handling during these various steps contemplates the integral formation of tie bars and external rails from the conductive sheet. The tie bars connect the lead frame to the external rails. The external rails provide a structure for machinery to grasp and maintain alignment of the lead frame while traveling down a manufacturing assembly line. To obtain a stable attachment to the lead frame, the tie bars must be connected to a solid structure on the lead frame which is typically done by extending the tie bars along the centerline of the lead frame in DIP and SO packages. Following molding of the lead frame and components to form an exterior package, the external rails are trimmed by cutting the tie bar at the edge of the molding and the leads at appropriate places. This leaves the cut-off end of the tie bar exposed through the molding and electrically coupled to one of the internal circuits.
When a pair of isolated circuits are to be encapsulated in a single package, the presence of the tie bar could pose a problem for maintaining a minimum spacing between primary and secondary circuit when one of the circuits requires a number of pins. By way of example, if a conventional width DIP or SO package is used for high voltage isolated circuits, leads for the primary and secondary circuits may not be positioned at opposite sides of the same end if the tie bar is coupled to either circuit since the exposed end of the tie bar would be too close to the opposing circuit for UL approval.
FIG. 1b shows a prior art small form factor package lead frame for isolated circuits having center mounted tie bars 12 and 13. The figure shows the areas punched out of a conductive sheet 10 used for making lead frames. Since the center mounted tie bars 12 and 13 reduce the external spacing between pins, an unconventional scheme is utilized in order satisfy external spacing requirements. In this arrangement, two separate die attach pads 14 and 16 respectively are separated by a gap 18. A primary circuit is positioned on die attach pad 14 and a secondary circuit is positioned on die attach pad 16 where the die attach pads are separated by gap 18 to meet internal spacing requirements. Although this technique allows both internal and external spacing requirements to be met, it is inherently inflexible with respect to pin arrangements. For example, since tie bar 12 is connected to the primary circuit 14, the pins in the proximity of tie bar 12 must be associated with the primary circuit. Similarly, tie bar 13 is connected to the secondary circuit thus the pins in the proximity of tie bar 13 must be associated with the secondary circuit therefore limiting the package to particular pin configurations where a pin of an opposing circuit is not permitted near the tie bar of the other circuit.
Accordingly, an improved method and arrangement for supporting the lead frame during manufacturing of relatively high pin count isolated circuit packages that allow for flexible pin configurations, without jeopardizing UL spacing requirements would be desirable.